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Tuning of Color Contrast Signals to Visual Sensitivity Maxima of Tree Shrews by Three Bornean Highland Nepenthes Species

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Tuning of Color Contrast Signals to Visual Sensitivity Maxima of Tree Shrews by Three Bornean Highland Nepenthes Species Plant Signaling & Behavior ISSN: (Print) 1559-2324 (Online) Journal homepage: http://www.tandfonline.com/loi/kpsb20 Tuning of color contrast signals to visual sensitivity maxima of tree shrews by three Bornean highland Nepenthes species Jonathan A. Moran, Charles Clarke, Melinda Greenwood & Lijin Chin To cite this article: Jonathan A. Moran, Charles Clarke, Melinda Greenwood & Lijin Chin (2012) Tuning of color contrast signals to visual sensitivity maxima of tree shrews by three Bornean highland Nepenthes species, Plant Signaling & Behavior, 7:10, 1267-1270, DOI: 10.4161/ psb.21661 To link to this article: http://dx.doi.org/10.4161/psb.21661 Published online: 20 Aug 2012. Submit your article to this journal Article views: 179 View related articles Citing articles: 5 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=kpsb20 Download by: [77.232.15.45] Date: 29 January 2017, At: 05:33 SHORT COMMUNICATION Plant Signaling & Behavior 7:10, 1267-1270; October 2012; © 2012 Landes Bioscience Tuning of color contrast signals to visual sensitivity maxima of tree shrews by three Bornean highland Nepenthes species Jonathan A. Moran1,* Charles Clarke,2 Melinda Greenwood3 and Lijin Chin2 1School of Environment & Sustainability; Royal Roads University; Victoria, BC Canada; 2School of Science; Monash University Sunway Campus; Jalan Lagoon Selatan; Selangor, Malaysia; 3School of Biological Science; Monash University Clayton Campus; Melbourne, VIC Australia Keywords: mutualism, Nepenthes, pitcher plants, tree shrews, Tupaia, visual signaling T hree species of Nepenthes pitcher plants (Nepenthes rajah, Nepenthes lowii and Nepenthes macrophylla) specialize in harvesting nutrients from tree shrew excreta in their pitchers. In all three species, nectaries on the underside of the pitcher lid are the focus of the tree shrews’ attention. Tree shrews are dichromats, with visual sensitivity in the blue and green wavebands. All three Nepenthes species were shown to produce visual signals, in which the underside of the distribute. pitcher lid (the area of highest nectar production) stood out in high contrast to the adjacent area on the pitcher (i.e., was brighter), in the blue and green wavebands visible to the tree shrews. N. rajah showed the tightest degree of “tuning,” notably in the green waveband. Conversely, pitchers of Nepenthes burbidgeae, a typical insectivorous species sympatric not with N. rajah, did not produce a color pattern tuned to tree shrew sensitivity maxima. Do Many plant species use visual signals to attract animals for pur- mouth, allowing capture of any excreta.9,10 Here, the hypothesis poses such as pollination and seed dispersal.1-3 For example, to was tested that pitchers of these species produce color signals that maximize the probability of pollen transfer from anther to stigma, are tuned to the visual sensitivity maxima of their diurnal part- insect-pollinated flowers utilize sensory cues and morphological ner. Tree shrews are dichromats- they have two types of cone cell structures tailored to the physiology and behavior of the tar- in the retina, which are sensitive to blue light (S cones) and green get species.4,5 The traps of carnivorous plants have evolved in light (L cones).13,14 response to a similar evolutionary pressure to that which shaped Specifically, the degree of color contrast between the area the flowers of animal- pollinated plants: the necessity to attract of greatest nectar production (the lid) and the closest adjacent Bioscience. and retain the target animal at the site of maximum benefit to area of the pitcher proper (i.e., the “background”), was quanti- the plant. For example, in pitchers of Nepenthes rafflesiana Jack, fied. InN. rajah and N. macrophylla, this was the peristome; in the peristome (the collar-like structure surrounding the pitcher N. lowii, the peristome is vestigial,9 so the flared inner wall of the mouth, and the site of highest nectar production) stands out in pitcher was scanned instead (Fig. 1). For comparison, pitchers of high visual contrast to the pitcher body proper, in the UV, blue Nepenthes burbidgeae Hook. f. ex. Burb. were analyzed. This spe- and green wavebands. These correspond to the visual sensitivity cies is sympatric with N. rajah, but does not attract tree shrews, Landes maxima of many of the targeted insect prey taxa.6,7 However, not catching invertebrates in typical Nepenthes fashion (JM, pers. all Nepenthes deploy strictly carnivorous pitchers: recent studies obs. Fig. 1A). Thus, it was predicted that N. burbidgeae would have demonstrated that those of four Bornean species, Nepenthes not produce a color contrast signal tuned to tree shrew visual rafflesiana var. elongata (recently renamed Nepenthes baramen- sensitivity maxima. sis C. Clarke, J.A. Moran and Chi C. Lee8), Nepenthes rajah In all three of the tree shrew- attracting species, the pitcher lid Hook. f., Nepenthes macrophylla (Marabini) Jebb and Cheek was found to be brighter in the blue and green wavebands than ©2012 and Nepenthes lowii Hook. f., collect mammal excreta.9-12 The the adjacent area of peristome (inner wall of the pitcher body latter three species attract mountain tree shrews (Tupaia mon- for N. lowii), i.e., contrast values relative to background were tana); in return for the nectar provided by the pitchers, the ani- positive (Figs. 2 and 3). This was not the case for the typical, mals deposit excreta, from which the plants derive nitrogen.9,10 insectivorous N. burbidgeae, in which the lid was darker than In these three species, the lid, rather than the peristome, is the the peristome in both wavebands (i.e., negative contrast values; site of greatest nectar production, and is therefore the “target” Figs. 2 and 3). Thus, in the three tree shrew-specialized species, to which the tree shrews are required to orient themselves. This the lid, the site of the highest nectar production, was brighter than ensures that their hindquarters are positioned over the pitcher the adjacent background in the wavebands corresponding to the *Correspondence to: Jonathan Moran; Email: [email protected] Submitted: 05/28/12; Revised: 07/27/12; Accepted: 07/29/12 http://dx.doi.org/10.4161/psb.21661 www.landesbioscience.com Plant Signaling & Behavior 1267 Materials and Methods Fieldwork was undertaken in Sabah, Borneo in February 2011 at the Mesilau landslip, Mount Kinabalu [06° 03’ N, 116° 36’ E, 2050 m asl; N. rajah (n = 31 plants) and N. burbidgeae (n = 10)], and Mount Trus Madi [05° 33’ N, 116° 31’ E, 2500 m asl; N. mac- rophylla (n = 28) and N. lowii (n = 33)]. One mature, fully-opened pitcher was selected per plant. A reflectance scan,ca. 0.3 cm2 in area, was then taken of the underside of the lid, and an adjacent area of peristome (N. rajah, N. macrophylla and N. burbidgeae) or the flared inner wall at the upper part of the pitcher N.( lowii). Scans were taken under natural light at 1-nm intervals from 400 to 700 nm using a spectroradiometer (USB4000, Ocean Optics Inc.) and fiber optic probe (BIF200-UV/VIS, Ocean Optics). A second scan was then taken of a Spectralon® white standard (WS-1, Ocean Optics). The dark signal was subtracted from each reflectance measurement, after which the reflectance values were divided by the white standard values to provide a normalized reflectance index. Color contrast (C) between lid and peristome distribute. (inner wall for N. lowii) was calculated at 1-nm intervals across the scanned waveband as follows: not C = (Il - Ip) / Ip Do -2 -1 where Il and Ip are the reflected radiant flux values (W m nm ) for lid and peristome (inner wall for N. lowii), respectively, at a 15 Figure 1. Pitchers of species used in the study. A) Nepenthes burbidgeae. given wavelength. Tree shrew retinas contain two types of cone B) Nepenthes lowii. C) Nepenthes macrophylla. D) Nepenthes rajah. cell. The S cones (“Short waveband”) are sensitive to blue light (428 to 445 nm), whereas the L cones (“Long waveband”) are sensitive to green light (549 to 558 nm).13,14 No data exist for sensitivity maxima of the two types of cone present in the retina visual sensitivity in T. montana; available data from a congener, of the target animal.13,14 However, within this group, there were Tupaia belangeri, were used as a proxy. Area under curve (AUC) differences. N. rajah showed a contrast peak (ca. 560 nm), which was calculated for each contrast curve in the wavebands corre- Bioscience. corresponded almost exactly with the sensitivity region of the L sponding to the sensitivity regions of tree shrew S and L cones, cones in the tree shrew retina; N. macrophylla showed a similar using the trapezoidal rule.16 The sensitivity waveband of the S pattern, although the contrast peak occurred at a slightly shorter cones is 18 nm in width (428–445), whereas that of the L cones wavelength (ca. 535 nm) than the L cone sensitivity maximum. is 10 nm (549–558). Therefore, to allow meaningful compari- N. lowii showed the least tight “fit” between contrast peak and L son between the two, a correction factor of 1.8 was applied to all cone sensitivity, with the former occurring at a longer wavelength AUC values for the latter. Data were analyzed using SigmaPlot Landes (ca. 610 nm). For the waveband corresponding to the S cones, all v.12 (Systat Software Inc.). AUC data were tested for homosce- three species showed positive contrast values, although to a lesser dasticity and normality using Levene’s and Kolmogorov- degree than for the L cone waveband, i.e., the contrast signal was Smirnov/Lilliefors tests, respectively. As transformation failed more highly tuned to the latter, in all three species. In terms of to make the data meet the assumptions of a parametric test, the overall “tuning” of the visual signal to the visual sensitivity of tree nonparametric Kruskal-Wallis test was used in lieu of one-way shrews, the tightest tuning was exhibited by N.
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